Photonic crystals (PCs) are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which electromagnetic waves within a certain stop band wavelength range are totally reflected. The wavelengths of the stop band are dependent on the distance between the periodic modulations in the crystal. The reflected stop band wavelengths appear in the reflectance spectrum as a distinct reflectance peak known as a Bragg peak. The crystal may have a one-, two-, or three-dimensional periodic structure.
Because of the sensitivity of a PC, slight changes in the refractive index or lattice spacing results in detectable changes in the reflected light. This is particularly useful where the reflected light is in the visible range, allowing for sensors with a colour-based response if an analyte can modulate the refractive index or lattice spacing, or for colour display systems if such modulations can be effected by an electric or electrochemical stimulus. One example of such an application is given by Arsenault et al. in U.S. patent application Ser. No. 10/681,374, which is hereby incorporated herein by reference.
The self-assembly of monodisperse microspheres into close-packed structures allows the formation of colloidal PCs (CPCs) in the form of optical films (Busch et al., Phys. Rev. E, 58:3896, 1998; Xia et al., Adv. Mater., 12:693, 2000). They can be made from a plethora of materials, and their stop band wavelength ranges are highly sensitive to changes in the optical characteristics or the structure of the photonic crystal.
Another type of PC is the hydrogel-based PC, such as that described by Asher in U.S. Pat. No. 6,544,800. In the hydrogel CPC, a hydrogel holds charged nanoparticles in an ordered array. The nanoparticles are suspended in a non-close-packed array due to charge repulsion, but are not structurally connected.
PCs can also be made using a templating strategy, in which structures formed by self-assembled monodisperse microspheres are used as a template for an infiltrating material. When the template is removed, the result is a photonic crystal having an ordered array of voids. Such a templating strategy is disclosed in U.S. Pat. No. 6,261,469, the disclosure of which is hereby incorporated herein by reference. The photonic crystal disclosed in this reference is in block form, which may not be suitable in many applications.
Previous studies of deformable PCs have been on the deformation of non-close-packed spheres embedded in hydrogel or elastomer matrices (Holtz et al., Nature 389:829-832, 1997; Foulger et al., Advanced Materials 13:1898-1901, 2001; Haacke et al., U.S. Pat. Nos. 5,266,238 and 5,368,781, 1993; Asher et al., Journal of the Material Chemical Society 116:4997-4998, 1994; Jethmalani et al., Chemical Materials 8:2138-2146, 1996). All of these studies deal with non-porous solid materials, where a compression along one direction must be accompanied by an expansion along perpendicular directions to maintain a constant volume.
An optical device using such a material has been disclosed in U.S. Pat. No. 6,956,689, in which the lateral expansion, bulging, deformation, or distortion of a plastic photonic crystal when vertically compressed is used to vary the wavelength of light transmitted laterally through the crystal.
Haacke et al. in U.S. Pat. No. 5,266,238 disclose the use of a filter film using such a hydrogel PC. In that patent, absorbance of the filter film is shifted by applying tension to the hydrogel PC. The hydrogel PC is compressible, but the fragility of its structure hinders its application. Further, as explained above, such a PC exhibits lateral expansion or bulging upon vertical compression.
Known deformable PCs include PCs having microspheres embedded in a deformable matrix, such as a hydrogel. These exhibit lateral expansion or bulging when vertically compressed, to preserve the constant volume of the PC. Since PC devices are fashioned as a film on a substrate, lateral expansion is accompanied by delamination from the substrate, inhomogeneity in color shift, and/or unrelieved stress in the material leading to loss of longer-term stability.
It is desirable to have a PC that has a stable response to compression. In particular, a reversible response offers a greater variety of applications for the PC. Such a PC may be a thin film PC with high sensitivity to compressive forces. The PC may exhibit point-response—that is, compression in one portion of the PC would not affect any uncompressed portion.
It is also desirable to have a PC that can be used in anti-counterfeiting, anti-tampering, fingerprinting, or strain-sensing applications.